This lines gives an unique identifier for the WebRTC Media Stream (WMS) during the PeerConnection’s life. This identifier that will be used in the a=msid attributes for each m-line belonging to a specific Media Stream (in our case both audio and video m-lines). This means that the RTP media stream (identified by the SSRC field present in every RTP packet) belongs to that media stream and that it is a track of that media stream. It is an explicit association of an individual RTP media stream to the MediaStream WebRTC object. For more info about this refer to draft-ietf-mmusic-msid